Thursday, 20 November 2003: 11:30 AM
Fire history and stand scale dynamics of mixed conifer forests in the Lake Tahoe Basin
Prior to the advent of fire suppression (ca. 1870), recurring wildfires played an important role in shaping the structure and dynamics of mixed conifer forests in the central Sierra Nevada. Variation in patterns of fire occurrence and severity are thought to have been controlled primarily by temporally and spatially variable fuel mosaics that are related to vegetation structure, stand development, and the time since the last fire. However, the dominant controls of fire regimes and vegetation dynamics are likely to vary at different spatial and temporal scales. In this study, we focused on the forest stand scale to assess the degree to which fire regimes and forest structure interact to maintain mixed conifer forests over time and to assess how stand characteristics have changed since fire suppression. Stand structural characteristics (i.e., species composition, size, age, and spatial structure) were sampled in 0.5-ha plots placed in 12 mixed conifer forest stands on the west shore of Lake Tahoe. Fire chronologies were developed using cross-dated fire scarred wood samples located in or near each plot. Forest spatial structure was assessed using Ripley’s K(d) statistic. Three mixed conifer forest types were identified based on cluster analysis of tree species’ importance values and were associated with different topographic settings. Although median fire return intervals (FRI) were longest (15 yrs) in red fir dominated stands and shortest (10 yrs) in Jeffrey pine dominated stands, the differences between plots were not statistically significant. Generally, trees were significantly clustered across a range of spatial scales (28 - 1962 m^2) due to the high density of young trees (<100 yrs) that have established since the beginning of fire suppression. Trees established before ca. 1870 were significantly clustered at small to intermediate spatial scales (28 - 254 m^2), and while some clusters consisted of relatively even-aged trees, most were multi-aged, with tree ages ranging from 120-400 years. This evidence suggests that the frequent fires that burned though these stand were typically of low to moderate severity, leaving many survivors over time. The similarities of fire history and the age, size and spatial structures among stands in different environmental settings and with variable species composition supports the view that, at the stand level, fire and forest structure interactions were important in the maintenance of mixed conifer forests. With the removal of fire, the structural diversity created by these interactions has been greatly reduced.